The bending of spacetime in General Relativity (GR) provides tantalizing possibilities of spacetime geometries that inspire one's imagination: black holes, wormholes and time machines associated with the closed time-like curves. Increasingly precise astronomical observations confirm the existence of black holes in our Universe. These include the detections of gravitational waves from two colliding black holes [1]}, [2]} and the recent direct photon picture of the black hole shadow [3]}. Although there is not yet any evidence of wormholes and time machines, these research subjects have continued to attract attentions in the GR community [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}. In addition to its traditional role as a tool for space and time travel, a wormhole may play an important role in quantum information through quantum entanglement [15]}, [16]}, [17]}, [18]}, [19]}, [20]}, [21]}, [22]}.
